In many applications, including the fields of fusion reactors, radio frequency systems, and telemetry systems, for example, it is often necessary to measure the magnitude of current flowing through circuits across which high voltages are applied, for example.
One particular example of such a system involves the Tokamak Fusion Test Reactor (TFTR) located in the Plasma Physics Laboratory, at Princeton University. In operating the TFTR, it is necessary to monitor the magnitude of the current in the neutral beam gradient grid. Typically, currents of less than 1.0 ampere ranging from the low to high milliamperes in magnitude, at voltages of about 100.0 kilovolts DC, must be measured. Current monitors utilized must be capable of withstanding high levels of transient voltage, and provide a means of safely measuring relatively low-magnitudes of current in the presence of relatively high-voltage levels, without requiring the use of high-voltage isolated power supplies or batteries.
There have been many systems developed in the prior art for measuring the magnitude of current flowing in circuits across which a relatively high voltage is applied. A number of these prior systems are discussed below.
In Summerhayes U.S. Pat. No. 4,070,572 (hereinafter Summerhayes), as shown in FIG. 1, a current monitor circuit for an ac high voltage line 100 includes a shunt 101 which is used to derive an alternating current signal proportional to the current flow in the ac line 100. The signal is coupled through a resistor R102 and capacitor C106 to the inverting terminal of an amplifier A103, the output terminal of which is connected to the base electrode of a current amplifying transistor Q. The PNP transistor Q delivers current to an LED 105, which responds by emitting a light or optical signal that is proportional in intensity to the magnitude of current flowing through the ac line 100. A photodiode PD1, connected across the inverting and noninverting terminals of the amplifier A103, provides negative feedback for causing the output of the light-emitting diode to vary as a linear function of the magnitude of the input signal into the amplifier A103. The light output from the LED 105 is coupled through a fiber optic cable 110 to a photo detecting diode PD2 of a remotely located receiver. Variable negative feedback is provided via a variable resistance cell R202 connected between the inverting and output terminals of amplifier A201, and to the output terminal of amplifier A203. The latter has an inverting input terminal connected to the output terminal of amplifier A201. A low pass filter circuit is provided by the combination of resistors R204, R205, and the capacitor C206. A dc reference voltage is applied to the noninverting terminal of amplifier A203, whereby this reference voltage has the same level as a DC reference voltage applied to one end of resistor R104 of the transmitter circuit. These reference voltages, and the variable negative feedback provided by R202 compensate for drift in the characteristics of the photodiodes PD1 and PD2.
Summerhayes also teaches in FIG. 2 an alternative receiver that includes an analog multiplier element M202 for controlling the gain of the receiver with less distortion than can be provided using the variable resistance cell R202 of FIG. 1. Yet another alternative receiver circuit is shown in FIG. 3, whereby an error signal derived from a comparison of the dc reference signal V.sub.ref2 with the dc component in the received signal relative to amplifier A203, is fed to a motor 210 for driving a lead screw 212 to vary the distance between the end of the fiber optic cable 110a and the photodiode PD2. A power supply 106 that derives power from the ac high voltage line 100 via a transformer coupling 107 is also included.
Milkovic U.S. Pat. No. 4,754,219 (hereinafter Milkovic), teaches the use of thin film ferromagnetic current sensors for detecting the current flowing through an ac line, and providing electrical isolation between the power conductors and the current sensing circuitry. The current signals are amplified and processed for providing the watt hours of power flowing through the three-phase conductors shown in FIG. 2 of Milkovic.
Nguyen Tan Tai et al. U.S. Pat. No. 4,780,668, discloses a circuit for measuring current signals of substantially small magnitude at voltages up to several kilovolts. The low level signals are amplified at the high potential, converting the signal to a potential at ground reference, for measuring the level of the same which is proportional to the current flowing through the main conductor of the system being monitored.